Microphones are ubiquitous on many devices used by individuals, including computers, tablets, smart phones, and many other consumer devices. Generally speaking, a microphone is an electroacoustic transducer that produces an electrical signal in response to deflection of a portion (e.g., a membrane or other structure) of a microphone caused by sound incident upon the microphone. To process audio signals generated by a microphone, microphones are often coupled to an audio system. However, many traditional audio system topologies may have disadvantages, as is illustrated with reference to FIG. 1.
FIG. 1 illustrates a block diagram of selected components of an example audio system 100, as is known in the art. As shown in FIG. 1, audio system 100 may include an analog signal path portion comprising bias voltage source 102, a microphone transducer 104, bias resistor 107, analog pre-amplifier 108, and a digital path portion comprising an analog-to-digital converter (ADC) 110, a driver 112, and a digital audio processor 114.
Bias voltage source 102 may comprise any suitable system, device, or apparatus configured to supply microphone transducer 104 with a direct-current bias voltage VBIAS, such that microphone transducer 104 may generate an electrical audio signal. Microphone transducer 104 may comprise any suitable system, device, or apparatus configured to convert sound incident at microphone transducer 104 to an electrical signal, wherein such sound is converted to an electrical analog input signal using a diaphragm or membrane having an electrical capacitance (modeled as variable capacitor 106 in FIG. 1) that varies as based on sonic vibrations received at the diaphragm or membrane. Microphone transducer 104 may include an electrostatic microphone, a condenser microphone, an electret microphone, a microelectromechanical systems (MEMs) microphone, or any other suitable capacitive microphone. As shown in FIG. 1, the bias circuit for microphone transducer 104 may also include bias resistor 107 coupled between microphone transducer 104 and a ground voltage.
Pre-amplifier 108 may receive the analog input signal output from microphone transducer 104 and may comprise any suitable system, device, or apparatus configured to condition the analog audio signal for processing by ADC 110.
ADC 110 may receive a pre-amplified analog audio signal output from pre-amplifier 108, and may comprise any suitable system, device, or apparatus configured to convert the pre-amplified analog audio signal received at its input to a digital signal representative of the analog audio signal generated by microphone transducer 104. ADC 110 may itself include one or more components (e.g., delta-sigma modulator, decimator, etc.) for carrying out the functionality of ADC 110. Driver 112 may receive the digital signal output by ADC 110 and may comprise any suitable system, device, or apparatus configured to condition such digital signal (e.g., encoding into Audio Engineering Society/European Broadcasting Union (AES/EBU), Sony/Philips Digital Interface Format (S/PDIF), or other suitable audio interface standards), in the process generating a digitized microphone signal for transmission over a bus to digital audio processor 114.
Once converted to the digitized microphone signal, the digitized microphone signal may be transmitted over significantly longer distances without being susceptible to noise as compared to an analog transmission over the same distance. In some embodiments, one or more of bias voltage source 102, pre-amplifier 108, ADC 110, and driver 112 may be disposed in close proximity with microphone transducer 104 to ensure that the lengths of the analog signal transmission lines are relatively short to minimize the amount of noise that can be picked up on such analog output lines carrying analog signals. For example, in some embodiments, one or more of bias voltage source 102, microphone transducer 104, pre-amplifier 108, ADC 110, and driver 112 may be formed on the same integrated circuit die or substrate.
Digital audio processor 114 may comprise any suitable system, device, or apparatus configured to process the digitized microphone signal for use in a digital audio system. For example, digital audio processor 114 may comprise a microprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), or any other device configured to interpret and/or execute program instructions and/or process data, such as the digitized microphone signal output by driver 112.
Despite the various advantages of digital microphone systems such as those shown in FIG. 1, such digital microphone systems may have disadvantages. For example, bias resistor 107 is often implemented using a back-to-back poly diode resistor. Such poly diode resistors are often susceptible to a tremendous variation with temperature, sometimes on the order of magnitude of a factor of 1000. For example, an example poly diode resistor may have a resistance of 26 TΩ at −20° C., a resistance of 800 GΩ at 25° C., and a resistance of 25 GΩ at 75° C. Because the bias resistance is so high, especially at low temperatures, even extremely small leakage currents may result in a direct-current (DC) offset voltage at the input to pre-amplifier 108, which may in turn lead to loss of measurement sensitivity, amplifier overload, and other negative effects.
One existing solution to overcome these disadvantages has been to include, interfaced between microphone transducer 104 and pre-amplifier 108, a high-pass filter to filter out such negative characteristics. However, such high-pass filters may reduce leakage-induced DC offsets that cause amplifier overload, but do not help with the problem of microphone sensitivity change caused by leakage. In addition, such high-pass filters may introduce extra noise in the system and have settling transients.